Thermal energy storage system including a vessel having hot and cold liquid portions separated by floating piston

ABSTRACT

A thermal energy storage system comprising a working fluid to store and transfer thermal energy between a heat source and a thermal load and a vessel to store the working fluid. The vessel has an interior region and a floating separator piston in the interior region to separate a hot portion from a cold portion of the working fluid. There is a first manifold thermally coupled to an output of the heat source and to an input of the thermal load and fluidly coupled to the interior region of the vessel and a second manifold thermally coupled to an input of the heat source and an output of the thermal load and fluidly coupled to the interior region of the vessel. There is a controller configured to maintain the working fluid in a liquid state.

TECHNICAL FIELD

The present invention relates to thermal energy storage systems, andmore particularly to a thermal energy storage system having a vesseldivided into hot and cold sides by floating separator pistons andincluding a controller configured to maintain the working fluid in aliquid state.

BACKGROUND ART

Research and development of large-scale renewable energy storage hasbeen rapidly growing with an increasing global demand for more energyfrom sources that reduce the output of greenhouse gas emissions. Asignificant limitation of renewable energy sources, such as wind andsolar, are their dependence on the weather and, in the case of solar,sunlight. Another drawback is their inability to store anddeliver/dispatch power when required. While several forms of energystorage are currently commercially available, new long-term andshort-term storage concepts are continually being developed and improvedupon to decrease capital costs and increase energy conversionefficiencies.

Thermal energy storage (TES) is one form of energy storage that may beused in conjunction with renewable energy sources. TES allows excessthermal energy to be stored and used at varying scales ranging fromindividual homes and buildings to electric grid scale energy storagethat may produce megawatts of power. The thermal energy for storage mayinclude, for example, heat produced with heat pumps from off-peak, lowercost electric power, a practice called peak shaving; heat from combinedheat and power (CHP) power plants; heat produced by renewable electricalenergy; and waste heat from industrial processes.

One promising source of heat that may be used in conjunction with TES isa concentrating solar power (CSP) system, which uses reflective surfacesto concentrate sunlight onto a small area, where it is absorbed andconverted to heat. Concentrators can increase the power flux of sunlighthundreds of times and can be used to heat a fluid such as water. Theheated water may be stored and later used to generate steam to spin aturbine for electricity production. The heated water may also be used asa heating source.

There are drawbacks for TES systems, as they typically have high initialcosts for material and installation and a long ROI. In addition, theyrequire a lot of space, are not yet highly efficient, and are notscalable. Therefore, there exists a need for a TES system with a reducedcost and footprint, but with increased efficiency and with the abilityto be easily scaled.

SUMMARY OF THE EMBODIMENTS

In accordance with one embodiment of the disclosure, there is a thermalenergy storage system for storing thermal energy produced by a heatsource and for supplying the thermal energy to a thermal load. Thethermal energy storage system includes a working fluid configured tostore the thermal energy and transfer the thermal energy between theheat source and the thermal load and a vessel configured to store theworking fluid. The vessel has a first end, a second end, an interiorregion, and a floating piston located in the interior region to separatea hot portion of the working fluid towards the first end from a coldportion of the working fluid towards the second end. There is a firstmanifold thermally coupled to an output of the heat source and to aninput of the thermal load and fluidly coupled to the interior regionproximate the first end of the vessel. There is a second manifoldthermally coupled to an input of the heat source and an output of thethermal load and fluidly coupled to the interior region proximate thesecond end of the vessel. There is a controller configured to maintainthe working fluid in a liquid state.

In some embodiments, one or more of the following features may beincluded. The first manifold may be thermally coupled to the output ofthe heat source by way of one of a first heat exchanger or a directfluid coupling and the first manifold may be thermally coupled to theinput of the thermal load by way of one of a second heat exchanger or adirect fluid coupling. The second manifold may be thermally coupled tothe input of the heat source by way of one of a second heat exchanger ora direct fluid coupling and the second manifold may be thermally coupledto the output of the thermal load by way of one of the second heatexchanger or a direct fluid coupling. The thermal heat source may be oneor more of a concentrating solar power system, a geothermal system, abiomass system, a waste-to-energy system, and an industrial heatrecovery system and the thermal load may be one or more of a heat engineand/or an industrial process heat load. The working fluid may compriseone or more of water, water mixed with one or more additives, oil,refrigerants, and molten salts. The working fluid may be water and thecontroller may be configured to maintain the hot portion of the workingfluid at a temperature from about 200 to 360 degrees C. and to maintainthe cold portion of the working fluid at a temperature from about 80-170degrees C. The controller may be configured to maintain a pressure ofthe working fluid between 225 psi (15 bar) and 2700 psi (190 bar) tomaintain a liquid state. The vessel may be disposed in a substantiallyhorizontal direction relative to a surface on which the thermal energystorage system is disposed and the first end of the vessel may bepositioned at a first height above the surface on which the thermalenergy storage system is disposed and the second end of the vessel maybe positioned at a second height above the surface. The first height maybe greater than the second height and the difference between the firstheight and the second height results in the at least one vessel may beoriented at an angle of between 0.25 and 2 degrees relative to thesurface.

In other embodiments, one or more of the following features may beincluded. The vessel may comprise steel and it may be insulated. Thevessel may comprise a plurality of vessel sections joined together viawelding and each of the vessel sections may be 40 to 80 feet in lengthand 24 to 48 inches in diameter. There may further be included a firstpump connected between the heat source and the vessel to circulate theworking fluid between the heat source and the vessel and a second pumpconnected between the vessels and the thermal load to circulate theworking fluid between the vessel and the thermal load. There may furtherbe included a thermal expansion system fluidly coupled to one of thefirst or second manifolds to accommodate a change in working fluidvolume. The thermal expansion system may include an expansion tank andan injection pump and the controller may direct the working fluid fromone of the first or second manifolds into the expansion tank when thepressure of the working fluid exceeds a setpoint pressure. Thecontroller may cause the injection pump to drive the working fluid fromthe expansion tank to one of the first or second manifolds when thepressure of the working fluid falls below the setpoint pressure tomaintain the working fluid in the liquid state. The vessel may compriseone or more of pipes, tubes, or conduits. The controller may beconfigured to control movement of the floating separator piston. Thefloating piston may comprise a piston body having a first end, a secondend, and a central region and a compressible member which is disposed inthe central region of the floating separator piston and which isconfigured to engage with an inner surface the vessel. The compressiblemember may be compressible to a thickness of between 25-75% of itsoriginal thickness. The compressible member may have a length of atleast 50-90% of a length of the piston body. The compressible member mayinclude one or more of Kevlar, glass, a ferrofluid, or a metallicmaterial. The compressible member may have a porosity level that resultsin a thermal loss due to leakage of the working fluid from the first endof the piston to the second end of the piston of no more than 5% of anoverall thermal loss in the thermal energy storage system. Thecompressible member may engage with an inner surface the vessel with anamount of friction that allows movement of the floating separator pistonin the vessel with a pressure difference of not more than 10 psi betweenthe first end of the piston to the second end of the piston. The innersurface of the vessel may have a variable roughness. The floating pistonmay have a neutral buoyancy state in the working fluid. The piston bodymay include an internal chamber which may be evacuated to create avacuum or it may include one or more of air, a nonreactive gas, a foamedglass or a metal. The vessel may be disposed in a substantiallyhorizontal direction relative to a surface on which the thermal energystorage system is disposed.

In accordance with another embodiment of the disclosure, there is athermal energy storage system for storing thermal energy produced by aheat source and for supplying the thermal energy to a thermal load. Thethermal energy storage system includes a working fluid configured tostore the thermal energy and transfer the thermal energy between theheat source and the thermal load. There is a vessel configured to storethe working fluid. The vessel has a first end, a second end, an interiorregion, and a floating piston located in the interior region to separatea hot portion of the working fluid towards the first end from a coldportion of the working fluid towards the second end. There is a firstmanifold thermally coupled to an output of the heat source and to aninput of the thermal load and fluidly coupled to the interior regionproximate the first end of the vessel. There is a second manifoldthermally coupled to an input of the heat source and an output of thethermal load and fluidly coupled to the interior region proximate thesecond end of the vessel. There is a controller configured to maintainthe working fluid in a liquid state. The vessel is disposed in asubstantially horizontal direction relative to a surface on which thethermal energy storage system is disposed. The first end of the vesselis positioned at a first height above a surface on which the thermalenergy storage system is disposed and the second end of the vessel ispositioned at a second height above the surface; and wherein the firstheight is greater than the second height.

In accordance with yet another embodiment of the disclosure, there is athermal energy storage system for storing thermal energy produced by aheat source and for supplying the thermal energy to a thermal load. Thethermal energy storage system includes a working fluid configured tostore the thermal energy and transfer the thermal energy between theheat source and the thermal load. There is a vessel configured to storethe working fluid. The vessel has a first end, a second end, an interiorregion, and a floating piston located in the interior region to separatea hot portion of the working fluid towards the first end from a coldportion of the working fluid towards the second end. There is a firstmanifold thermally coupled to an output of the heat source and to aninput of the thermal load and fluidly coupled to the interior regionproximate the first end of the vessel. There is a second manifoldthermally coupled to an input of the heat source and an output of thethermal load and fluidly coupled to the interior region proximate thesecond end of the vessel. There is a controller configured to maintainthe working fluid in a liquid state. The floating piston comprises apiston body having a first end, a second end, a central region; and acompressible member which is disposed in the central region of thepiston and which is configured to engage with an inner surface thevessel. The compressible member has a porosity level that results in athermal loss due to leakage of the working fluid from the first end ofthe piston to the second end of the piston of no more than 5% of anoverall thermal loss in the thermal energy storage system. Thecompressible member engages with an inner surface the vessel with anamount of friction that allows movement of the floating separator pistonin the vessel with a pressure difference of not more than 10 psi betweenthe first end of the piston to the second end of the piston.

In accordance with a further another embodiment of the disclosure, thereis a thermal energy storage system for storing thermal energy producedby a heat source and for supplying the thermal energy to a thermal load.The thermal energy storage system includes a working fluid configured tostore the thermal energy and transfer the thermal energy between theheat source and the thermal load. There is a vessel configured to storethe working fluid. The vessel has a first end, a second end, an interiorregion, and a floating piston located in the interior region to separatea hot portion of the working fluid towards the first end from a coldportion of the working fluid towards the second end. There is a firstmanifold thermally coupled to an output of the heat source and to aninput of the thermal load and fluidly coupled to the interior regionproximate the first end of the vessel. There is a second manifoldthermally coupled to an input of the heat source and an output of thethermal load and fluidly coupled to the interior region proximate thesecond end of the vessel. There is a controller configured to maintainthe working fluid in a liquid state. The first manifold is thermallycoupled to the output of the heat source by way of a direct fluidcoupling; and wherein the second manifold is thermally coupled to theinput of the heat source by way of a direct fluid coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of embodiments will be more readily understood byreference to the following detailed description, taken with reference tothe accompanying drawings, in which:

FIG. 1 shows a renewable and dispatchable energy generation system,including a thermal storage system in accordance with an embodiment ofthe present disclosure incorporated with a solar energy field as theheat source and an Organic Rankine Cycle (ORC) heat engine as thethermal load;

FIG. 2 shows a renewable and dispatchable energy generation system,including a thermal storage system in accordance with a relatedembodiment of the present disclosure depicting optional types of heatsources and thermal loads;

FIG. 3 shows a thermal storage system in accordance with a relatedembodiment of the present disclosure;

FIG. 3A shows use of an inclined pipe for the thermal storage system ofFIG. 3 in accordance with an aspect of the present disclosure;

FIG. 4 shows a thermal storage system coupled with a concentrating solarpower (CSP) heat source using a common working fluid in accordance witha related embodiment of the present disclosure;

FIG. 5 shows a thermal storage system coupled with a CSP heat sourceusing a heat exchanger to transfer thermal energy to the storage systemin accordance with a related embodiment of the present disclosure;

FIG. 6 shows a renewable and dispatchable energy generation system,including a thermal storage system coupled with a CSP heat source and anORC heat engine using a common working fluid in accordance with arelated embodiment of the present disclosure;

FIG. 7 shows a piping and instrumentation diagram of a thermal storagesystem in accordance with a related embodiment of the presentdisclosure;

FIG. 8 shows an example of a pipe service or access port in accordancewith an aspect of the present disclosure;

FIG. 9A shows an exemplary floating separator piston in accordance withembodiments of the present disclosure;

FIG. 9B shows an exemplary floating separator piston in accordance withanother embodiments of the present disclosure, including aframe/armature for the compressible seal;

9C shows the body of the exemplary floating separator piston of FIG. 9Bin cross-section;

FIG. 10 shows exemplary segmented piston rings in accordance withaspects of the present disclosure;

FIG. 11 shows a cross-sectional view of the exemplary floating separatorpiston of FIG. 9 ;

FIG. 12 shows an alternative floating separator piston in accordancewith an aspect of the present disclosure;

FIG. 13 shows another alternative floating separator piston inaccordance with an aspect of the present disclosure;

FIG. 14 shows yet another alternative floating separator piston inaccordance with an aspect of the present disclosure;

FIG. 15 shows a further alternative floating separator piston inaccordance with an aspect of the present disclosure;

FIG. 16 shows a state diagram for a control system in accordance with anaspect of the present disclosure;

FIG. 17 shows an example of pump and control valve actions for severalsystem states in accordance with an embodiment of the present invention;and

FIG. 18 shows an example of control system logic in accordance with anaspect of the present disclosure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

There is described herein a low cost, bulk thermal energy storage systemhaving a plurality of horizontally disposed parallel connected vessels,each containing a floating separator piston to separate the vessels intohot and cold sides or sections of the working fluid, which may be water.The working fluid may be pressurized and maintained in the liquid phase.The working fluid in the storage system may be heated/charged by varioustypes of heat sources and it may be used/discharged to drive varioustypes of thermal loads.

The term “vessels” used herein includes various types of pipes, tubes,conduits, and the like and each of these terms may be usedinterchangeably. One exemplary form of pipe which is suitable for usewith the thermal energy system described in this disclosure is naturalgas pipeline. This is due to its ease of transport, its ability to storepressurized fluids, and its low cost and high reliability. However, thethermal energy storage system herein may be used with any other suitablevessel type.

The term “floating” as used herein with respect to the separator pistonsdisposed within vessels of the storage systems of this disclosure, meansthat the separator pistons do not have fixed mechanical connections tothe interior of the vessels.

For purposes of simplifying the description of the thermal storagesystem herein, it is described in conjunction with a CSP generationsystem as the heat source and an ORC heat engine as the thermal load toproduce electricity. Combining the CSP system and ORC heat engine withthe thermal storage system of this disclosure, provides a low cost, longduration, fully dispatchable renewable energy generation system with theability to support the connected customer's electrical demand duringperiods of little or no solar insolation. The inexpensive storage mediadescribed herein allows for economic long duration storage capacity,thus enabling a practical renewable based power system that can supplycustomer electrical loads around the clock.

It should be noted, however, that the thermal storage system describedherein can be used to store energy from various types of heat sources,such as biomass, geothermal, heat recovery and work to energy systems.The stored heat may be used to operate various types of a thermal loads,including an ORC heat engine or other closed circuit gas turbines, aStirling heat engine, steam turbine, or as a source of industrialprocess heat for a wide variety of applications.

Before describing specific embodiments and configurations of the thermalenergy storage system of this disclosure, we provide descriptions ofcertain common aspects of the system. These include types of workingfluids, operating temperatures and pressures, types and dimensions ofpipes which may be used for storing the working fluid, thermal storagecapacity, and certain functional aspects of the floating separatorpiston.

The thermal storage system herein may use long, insulated pipes(typically made of steel pipe sections welded together), separated intohot and cold sides by loose-fitting floating separator pistons which aredisposed and travel longitudinally in the interior of the pipes. Thehigh and low temperature sides of the thermal storage system can beoptimized for specific applications based on the heat sourcecharacteristics and the temperature range of the thermal energyload/output device utilizing the stored thermal energy.

In the case of a generation system which utilizes a CSP system to storeor charge the thermal storage system of this disclosure and an ORC heatengine to use the stored thermal energy to produce electricity, a highside temperature of approximately 280-300° C. and a low side temperatureof 150-170° C. would be typical when using water as the working fluid.More generally, for other types of heat sources and thermal loads,typical system conditions for a thermal storage system according to thisdisclosure would range from of 200-360° C. on the hot side, and 80-170°C. on the cold side. A large temperature differential between the hotand cold sides of the storage system preferably allows a higher energystorage capacity for a given storage system working fluid volume.

The thermal storage system may be pressurized to keep the water used asthe working fluid fully liquid under all operating conditions. Asexamples, a thermal storage system with a high temperature sideoperating at 200° C. would require a system pressure of above 225.3 psi(15.53 bar) to maintain a liquid state, whereas a system operating at300° C. would require a minimum operating pressure of 1245.5 psi (85.9bar). In general, the pressure range would typically be between 225 psi(15 bar) and 2700 psi (190 bar) to maintain a liquid state.

The use of a very low-cost heat transfer fluid (HTF), also referred toas working fluid, and thermal storage are key enablers to low life-cyclecost of energy (COE) solar CSP. Various types of working fluids may beused, such as water with or without additives, oil, refrigerants, moltensalts, and other possible materials. Water may be the optimal choice fora working fluid, due to its low cost, high heat capacity, zero toxicity,low reactivity, and excellent fluid dynamics. By operating the thermalstorage system over a broad temperature range, advantage can be taken ofthe high thermal capacitance of water to achieve high thermal-storagedensities, therefore eliminating the need to change phase or add thecomplexity of steam accumulation.

While other working fluids may be used, they do have drawbacks. Thermaloil may operate at similar temperatures to water, but oil cost prohibitscost-effective bulk storage, and bulk thermal oils are a potentialhazard. Molten salt freezes at typically about 220° C., is corrosive,and is costlier than water. Also, phase-change water storage(accumulators) may be used but they add complexity without substantialbenefits in storage density.

Natural gas pipeline has increasingly pushed the limits of low-costtransport and storage of pressurized fluids by using stronger steelsthat can be formed into seamless pipe or can be rolled and welded intopipes at low cost and high reliability. In the disclosed system, lowcost natural gas pipeline may be used to control system costs andprovide for easier transport and construction. The use of shorter pipesections, approximately 40 to 80 feet in length (approximately one-inchthickness one-meter diameter), enable easy transport via truck or rail.These shorter lengths may be welded together onsite to form long storagevessels (e.g. 300-1000 ft. in length) and they be connected in parallelvia end manifolds to form a storage field. In terms of pipe diameter,increases in pipe diameter require related increases in pipe wallthickness to maintain material. In practice, pipeline diameters of 24 to48 inches are preferable for this application, with current designstypically using a nominal 36 inch pipe outside diameter.

The use of commercially available pipeline sections to create thethermal storage vessels presents design challenges for the separatorpiston (discussed below) employed to maintain a thermal separationbetween the high and low temperature sides of each storage vessel. Thepipe sections must maintain their concentricity as they are beingtransported, handled in the field, and joined together. In addition, anyfield welds done to join adjacent pipe sections together may requirepost-weld grinding or similar operation to ensure proper operating ofthe separator piston as it moves past these joints during operation. Thefloating separator piston design with its attendant seal system must bedesigned to accommodate the standard pipe tolerances of the specifiedcommercial products used in this novel storage application as well asthe finished welds used to create the completed storage vessels.

According to this disclosure, the thermal storage system is designed touse the same vessels/pipes for both hot and cold working fluid storagein order to reduce costs and optimize system operation. Operationally,the system should achieve full depth of discharge and charge, i.e. thehot water should be fully expended when fully discharged, and vice versafor full charging. The storage system is “charged” by collecting heatenergy from a heat source (e.g. CSP) to its maximum temperature at thesystem pressure while being maintained in a liquid state and it is fully“discharged” by transferring the heat energy to the thermal load, e.g.an ORC heat engine. As thermal energy is collected and stored (i.e.charged), the piston in each pipe moves laterally to increase the hightemperature volume and decrease the volume of the low temperatureworking fluid in the storage system. When discharged, the pistons moveto increase the low temperature volume and decrease the high temperaturevolume

At the system design pressures, pressurized water can reach a mid-300°C. range before vaporizing. Thus, using a single vessel/pipe to storeboth hot and cold working fluid presents real challenges. According tothis disclosure, separation of hot and cold sides or sections in thepipes may be achieved with a floating separator piston. This piston mustmove freely while minimizing heat loss and thermal circulation betweenthe hot and cold sides. The storage system must also handle substantialthermal expansion of the water when heated. The pressure vessels/pipefield must be insulated to minimize heat loss. Rock or mineral wool orcalcium silicate insulation are viable candidates to provide sufficientinsulation and to minimize losses during daily cycling of the thermalstorage. Using pressurized water in steel pipes requires that the waterbe treated with anti-corrosion chemicals and filtered to minimizecorrosion and fouling. Makeup water may be introduced as needed afterfiltration by the circulation pumps.

In terms of thermal energy storage capacity, a 100 m section of theabove described pipe, with water as the working fluid, stores about 10.0MWhth. At 24% thermal-to-electrical efficiency for an exemplary ORC heatengine, six hours of storage for a 1 MWe plant output would requireabout 250 meters of pipeline for thermal storage. For behind-the-meter24/7 type applications, often fifteen or more hours of storage arerequired, which would be require over 600 meters of pipe. Such a longlength pipe may be hard to site in many cases. According to an aspect ofthe thermal storage system described herein, in order to allow for usewith more limited space available, the system may be configured withmultiple shorter pipe sections connected in parallel rather than asingle long length pipe. Notwithstanding the foregoing, various aspectsof the system disclosed herein are applicable to both parallel pipe andsingle pipe configurations.

Prior art thermal storage systems have utilized a vertically orientedstorage vessel to take advantage of the inherent thermocline effectwhere a hotter, less dense liquid sits above a heavier cooler volume. Ina vertical tank configuration, a separator piston can also be used toprovide increased separation and decreased heat transfer between the hotand cool sides of the energy storage system. With the horizontallyaligned storage system disclosed herein, the system does not takeadvantage of any inherent thermocline; however, it presents multiplepractical advantages. The advantages of the disclosed horizontalconfiguration include:

(1) Use of widely available and proven pipeline industry supply chain,construction methods, and infrastructure in a new configuration for thisdifferent application.

(2) the horizontal configuration integrates well with the overall layoutof a suitably sized CSP collection system, allowing the addition of longduration storage to a CSP plant with a relatively small increase infootprint.

(3) the storage system is entirely at ground level, easing system O&Meffort and costs.

(4) The storage system can be configured with multiple parallelpipelines, each equipped with a floating piston and operated inparallel. This configuration creates a modular system where individualsections in parallel can be isolated and shut down for service whileallowing the overall storage system to remain in service. Thisconfiguration also allows the expansion of total energy storage capacityover time by adding additional pipelines in parallel.

Various configurations and aspects of the thermal storage systemaccording to this disclosure are described in more detail below.

Thermal Storage System Configurations

FIG. 1 shows a renewable and dispatchable energy generation system 100in accordance with one embodiment of the present disclosure. System 100includes a thermal storage system 102 with parallel arrangement ofpipes, a heat source in the form of a solar field 106 (e.g. a CSP solarfield), and a power block for a thermal load, which may be an ORC heatengine 108. The bottom portion of FIG. 1 illustratively shows in moredetail a portion of the thermal storage system 102, including a parallelarrangement of pipe sections containing pressurized water divided intohot sides 102 a and 102 b and cold sides 102 c and 102 d separated byfloating separator pistons 104 a and 104 b. The hot sides 102 a and 102b of each of the pipes 102 are directly (or fluidly) connected to afirst manifold 112 and the cold sides 102 c and 102 d of each of thepipes are directly (or fluidly) connected to a second manifold 114.

While not clearly shown in this figure, the first manifold 112 may bethermally connected to the output of the CSP solar field 106 to receivethermal energy and it may be thermally connected to the input of theinput of the ORC system 108 to deliver the thermal energy to drive theORC system 108 to produce electricity. As described in more detailbelow, heated water may be stored in the hot sides 102 a/102 b of thepipes, which may be directly or fluidly coupled to the output of the CSPsolar field 106 and the input of the ORC system 108 by using a commonworking fluid or indirectly coupled via a heat exchanger.

The second manifold 114 may be thermally coupled to the input of the CSPsolar field 106 and it may be thermally coupled to the output of the ORCsystem 108. Thus, the ORC system returns cold water after extractingthermal energy from the hot water to produce electricity. As describedin more detail below, the cold water may be stored in the cold sides 102c/102 d of the pipes, which may be directly or fluidly coupled to theCSP solar field 106 and the ORC system 108 by using a common workingfluid or indirectly coupled via a heat exchanger.

The parallel configuration of pipes of thermal storage system 102 aredisposed in a substantially horizontal direction relative to a surfaceon which thermal energy system 102 is disposed. The number of pipesections in the parallel arrangement may be increased or decreased toachieve the amount of thermal storage required. Moreover, the lengths ofthe individual sections may be varied based on the amount of thermalstorage required and also on the footprint available for installation ofthe thermal energy storage system. It should be noted that to form thelonger pipe lengths, which may be required, shorter and more easilytransportable pipe sections (e.g. 40-80 ft.) may be welded togetheronsite to form desired pipe lengths, e.g. 300-1000 ft.

As will be described in more detail below, the storage pipes may bearranged with the hot sides 102 a and 102 b inclined slightly withrespect to the low temperature sides 102 c and 102 d to aid the floatingseparator pistons (104 a and 104 b) in separating the high and lowtemperature sides of the pipes.

It should be noted that each of the floating separator pistons, e.g.pistons 104 a and 104 b, may move independently, or their movement maybe controlled. To control the pistons, a controller of the thermalstorage system 102 may determine each piston position and then adjustthe flow of working fluid to each pipe to control movement of thepistons. This adjustment may, for example, be performed through controlvalves or pump speed. This action may be carried out in a number ofways. One method is to equip each parallel pipe with a flowmeter, anduse either a single system pump with flow control valves for eachparallel pipe or a separate variable speed pump for each parallel pipeto maintain equal flow to each pipe. This method is relatively easy toimplement, but it would need to account for potentially unequal pistonbypass leakage performance among the parallel pipes.

An alternate method of controlling movement of the floating separatorpiston in multiple storage pipes in parallel is to measure pipelinetemperature at multiple regular locations along the length of each pipeand to detect the high differential temperature that occurs on each sideof the floating separator piston. The piston location input signal canthen be used to control either flow control valves or variable speedpumps on each pipe circuit to manage the relative positions of thepistons, e.g. to keep the parallel pistons in synchronization or to movethem sequentially. This method is described in more detail below.

FIG. 2 shows a renewable and dispatchable energy storage system 200using a thermal storage system 202 with a parallel configuration ofpipes similar to that of parallel arrangement of FIG. 1 . In thisembodiment, it is illustrated that thermal storage system 202 may beintegrated with one or more types of heat sources 206. Examples for heatsources 206 are a CSP solar field (linear, dish, tower, or other solarconcentration systems), biomass/biofuel, geothermal energy, solar ponds,industrial heat recovery, and waste-to-energy. However, it is expresslycontemplated that any other type of heat source may be used to chargethe thermal storage system. It also illustrates that the thermal storagesystem may further be integrated with one or more types of thermal loads208. Examples for thermal loads 208 are an ORC heat engine or otherclosed circuit gas turbine, a Stirling heat engine, steam turbine, orprocess heat for industrial applications.

As with the thermal storage system 102 of FIG. 1 , thermal storagesystem 202 may be plumbed to directly or fluidly couple with heat source206 and heat load 208, using a common working fluid or the thermalstorage system 202 may be plumbed indirectly through heat exchangers tothermally couple with heat source 206 and heat load 208.

The number of parallel pipe sections may be increased or decreased toachieve the amount of thermal storage required. Moreover, the lengths ofthe individual sections may be varied based on the amount of thermalstorage required and also on the footprint available for installation ofthe thermal energy storage system. Additionally, the storage pipes maybe arranged with the hot sides inclined slightly with respect to the lowtemperature sides to aid the floating separator piston (not shown) inseparating the high and low temperature sections of the pipelines. And,it should be noted that each of the floating separator pistons may moveindependently, or their movement may be controlled.

In FIG. 3 there is shown a thermal storage system 300 in accordance withan alternative embodiment of the present disclosure. In this figure, theconnection of the heat source and thermal load is not explicitly shown,but it may be connected in the manner described above with regard tothermal storage systems 102 and 202 of FIGS. 1 and 2 , respectively.Such connections will also be described in more detail with regard toFIGS. 4-7 below.

Storage system 300 can be configured with multiple sets 302 a and 302 bof pipes plumbed together in parallel to increase storage capacity. Inthis example, each set includes four (4) parallel pipes; for set 302 athere are included parallel pipes 303 a-303 d and for set 302 b thereare included parallel pipes 303 e-303 h. Each parallel pipe is equippedwith a floating separator piston 304 that separates the cold and hotportions of the working fluid. Parallel pipes 303 a-303 d includefloating separator pistons 304 a-304 d and parallel pipes 303 e-303 hinclude floating separator pistons 304 e-304 h.

The hot sides of each of the pipes 303 a-303 h are fluidly coupled witha first or hot manifold 312 and the cold sides of each set of pipes areconnected to a cold manifold. In this example, the cold sides of thepipes 303 a-303 d of set 302 a are connected to second/cold manifold 314a and the cold sides of the pipes 303 e-303 h of set 302 b are connectedto third/cold manifold 314 b. Although not shown in detail, first/hotmanifold 312 is thermally connected to the output of a heat source toreceive thermal energy and it is also connected to the input of athermal load to provide thermal energy to the thermal load. Theconnections may be direct, using a common working fluid, or indirectusing a heat exchanger, such as heat exchanger 308 a. In addition, thesecond/cold manifold 314 a and the third cold manifold 314 b arethermally connected to the input of the heat source and to the output ofthe thermal load either directly, using a common working fluid, orindirectly using a heat exchanger, such as heat exchanger 308 b.

The pistons 304 a-304 h translate back and forth along their respectivepipes in response to the system pumps 306 moving fluid from one side ofthe pipes to the other side. The pistons 304 a-304 h may simply beallowed to react independently to the fluid flow and, therefore, theymay move at different rates during charge and discharge cycles of thestorage system. Alternatively, the pistons' respective positions can bemonitored and controlled to move in a coordinated way. This action maybe carried out in a number of ways. One method is to equip each parallelpipe 302 a-302 h with a flowmeter, and use either a single system pumpwith flow control valves for each parallel pipe or a separate variablespeed pump for each parallel pipe to maintain equal flow to each pipe.

An alternate method of controlling separator piston 304 a-304 h movementin storage pipes 302 a-302 h is to measure pipeline temperature atmultiple regular locations along the length of each pipe and to detectthe high differential temperature that occurs on each side of thefloating piston 304. This piston location input signal can then be usedto control either flow control valves or variable speed pumps on eachpipe circuit to manage the relative positions of the pistons, e.g. tokeep the parallel pistons in synchronization or to move themsequentially.

While the storage pipes are described as typically being substantiallyhorizontal in the thermal storage system disclosed herein, the pipes canalso be configured with a slight angle or incline relative to thehorizontal to augment the ability of the floating separator piston toseparate the high and low temperature sides, wherein the hightemperature side is positioned above the low temperature side.

As shown in FIG. 3A, pipe 303 a is depicted with its hot side endinstalled at a height H₁ above the surface on which thermal storagesystem 300 is installed, while the cold side end is installed at a lowerheight H₂, so that pipe 303 a is inclined at a slight angle, θ. Inpractice this angle, θ, would be quite small, typically from 0.25 to 2degrees from the horizontal position.

Any of the parallel pipe configurations disclosed herein may utilizeinclined pipes as described in FIG. 3A. Moreover, such an inclined pipemay be beneficial even in a storage system having a single pipe andusing a separator piston with a liquid phase, pressurized working fluid,according to an aspect of this disclosure.

FIG. 4 depicts how the thermal storage system according to thisdisclosure may use a common working fluid to directly and fluidly couplethe thermal storage system to the heat source. In FIG. 5 , describedbelow, it is shown how to indirectly couple the thermal storage to theheat source using a heat exchanger. In order to simplify thedescriptions of FIGS. 4 and 5 , thermal loads and their interconnectionsto the thermal storage systems are not shown.

Thermal storage system 400, FIG. 4 , may be directly charged from a heatsource 402, such as a CSP system, through a common working fluid. Theworking fluid is circulated through the CSP 402 by pump 406, where heatenergy is transferred to the working fluid, and then circulated andstored in the storage pipes 404 a and 404 b. High temperature workingfluid heated by the CSP 402 is transferred into the “hot” side 405 a and405 b of the thermal storage pipes 404 a and 404 b, respectively. The“cold” fluid from the low temperature or cold sides 407 a and 407 b ofstorage pipes 404 is transported also by pump 406, to the intake of CSP402 for heating. As this process progresses, the thermal separatorpistons 408 a and 408 b, located within each one of the storage pipes,translates from the hot side of the pipe to the cold side as the storagesystem stores more heat during charging. When the system transfers heatto a thermal load (not shown) while discharging the floating separatorpistons 408 a and 408 b move in the opposite direction to allow morestorage fluid for the cold fluid.

By using pressurized hot water as a working fluid to pass thermal energydirectly from the heat source (CSP 402) to the thermal storage system400, it eliminates the need for a heat exchanger between the heat sourceand the thermal storage system, simplifying the system design,potentially decreasing cost, and increasing overall system efficiency.It should be noted that the operating pressure required to keep thewater working fluid in the liquid state can be higher than the pressurerating of current off-the-shelf solar receiver tubes in CSP systems.Therefore, the solar receiver tubes must be specified with theappropriate pressure rating, increasing the receiver tube wall thicknessand material cost. However, in most cases this will be a favorabletradeoff, and the direct fluid coupling approach using a common workingfluid will be preferable. The direct fluid coupling may be applied toother types of heat sources as well.

Notwithstanding a preference for a direct fluid coupling, an indirectthermal connection between the heat source and the thermal storagesystem may be desired. Such a connection is depicted in FIG. 5 , wherethermal storage system 500 is indirectly charged from a heat sourcethrough a heat exchanger 510. In this configuration, the working fluidin the CSP 502 may include thermal oil or molten salt, while pressurizedwater would be used in the storage pipes 504 a and 504 b. A pump 506circulates a first working fluid in the heat source loop 512 through theCSP collectors 502, where the working fluid is heated, and then to theheat exchanger 510. From the heat exchanger the first working fluidreturns to the input of the CSP 502. The heat exchanger 510 transfersthe thermal energy in the first working fluid to the second workingfluid (i.e. pressurized water in the liquid phase) in the thermalstorage loop 514, where it is circulated by pump 516.

In the thermal storage loop, high temperature working is transferredinto the “hot” side 505 a and 505 b of the thermal storage pipes 504 aand 504 b, respectively. The “cold” fluid from the low temperature orcold side of the 507 a and 507 b of storage pipes 504 a and 504 b istransported to the cold return of heat exchanger 510. As this processprogresses, the thermal separator pistons 508 a and 508 b located withineach one of the storage pipes translates from one end of the pipe to theother as the storage system stores heat during charging and thentransfers heat to a thermal load (not shown) while discharging.

FIG. 6 shows a renewable and dispatchable energy generation system 600in accordance with this disclosure, which includes a heat source, i.e.CSP 620, thermal storage system 630 and a heat load, i.e. ORC heatengine 640. The solar concentrators 602 of CSP 620 and thermal storagesystem 630 are directly/fluidly coupled using a common working fluid(e.g. pressurized water in liquid phase) in the same manner as is system400 of FIG. 4 . The common working fluid is also used to transferthermal energy to the ORC heat engine via an integrated heat exchanger616 to extract thermal energy to generate electricity.

Thermal storage system 630 may be directly charged from heat source 620through a common working fluid. The working fluid is circulated throughthe CSP 602 by pump 606 (P1) where heat energy is transferred to theworking fluid and stored in the storage pipes 604 a and 604 b of thermalstorage system 630. High temperature working fluid heated by the CSP 602is transferred into the “hot” side 605 a and 605 b of the thermalstorage pipes 604 a and 604 b, respectively. The “cold” fluid from thelow temperature or cold side of the 607 a and 607 b of storage pipes 604is transported also by pump 606, to the intake of CSP 602 for heating.

When the thermal storage system is being charged, the control valves 619are used to circulate the working fluid only between the heat source 620and thermal storage system 630 and to isolate and separate the thermalload 640 from the thermal storage system 630. This is done by openingvalves 619 b (CV2) and 619 d (CV3) and closing valves 619 a (CV1), 619 c(CV4), and 619 e (CV5).

Thermal storage system 630 may be discharged from storage system 630 toORC heat engine 640 through the common working fluid. The working fluidis circulated by pump 618 (P2) from storage system 630 to heat exchanger616 of ORC heat engine 640. During discharge, the control valves 619 areused to circulate the working fluid only between the thermal storagesystem 630 and the ORC heat engine 640 and to isolate and separate theheat source 620 from the thermal storage system 630. This is done byopening valves 619 c (CV4) and 619 e (CV5) and closing valves 619 g(CV7), 619 b (CV2), and 619 d (CV5). As a result, high temperatureworking fluid is transferred from the “hot” side 605 a and 605 b of thethermal storage pipes 604 a and 604 b, respectively, to the input of theheat exchanger 616 in the ORC heat engine 640. The “cold” fluid passingout of the heat exchanger 616 is transferred back to the low temperatureor cold side 607 a and 607 b of storage pipes 604 a and 604 b.

The ORC heat engine includes a turbine 644 that receives a heated fluid(e.g. steam) heated by heat exchanger 616 which spins the turbine anddrives generator 646. The generator 646 produces electricity. The fluidoutput by turbine 642 is cooled by cooler 646, which could be a wet ordry cooler, and returned to the input of heat exchanger 616. The fluidis circulated in the ORC heat engine loop by pump 648.

Also shown in FIG. 6 is expansion system 614 to account for changes influid density and therefore total fluid volume as the thermal storagesystem 630 goes through charge and discharge cycles. Expansion system614 includes a control valve 619 f (CV6), pump 621 (P3), an expansiontank 622, and cooler 623. When fully charged, the fluid in the storagepipes 604 a and 604 b is at a higher temperature condition, andtherefore the pressurized water is at a lower density and requiresadditional system volume. The thermal expansion system 614 allows thestorage system 630 to operate without water loss and replenishment as itis charged and discharged. Alternatively, the thermal storage pipes 604a and 604 b may be sized to be only partially filled at the fullydischarged condition to accommodate the variable fluid density andvolume difference between the charged and discharged state of thesystem.

There is also shown a controller 650 which may control the operation ofthe various valves and pumps in order to charge and discharge thestorage system 630 as is described in more detail under the “storagesystem control” section below. The controller 650 may comprise anindustrial PLC, remote I/O, and supporting components to provide safetyand control functionality. The controller 650 may interact with aseparate or integrated higher level controller which may provide overallcontrol of the heat source 620, storage system 630 and heat load 640.The operation of the higher level controller is described below in moredetail with regard to FIGS. 16-18 .

FIG. 7 shows a piping and instrumentation diagram of a thermal storagesystem 700 in accordance with a related embodiment of the presentdisclosure. While not shown in this figure, the thermal storage system700 may be interconnected to a CSP heat source and an ORC heat engineheat load. Thermal storage system 700 includes storage pipes 702 and 704to store the working fluid, which pipes are connected in parallel tofirst/hot manifold 706 and second/cold manifold 708.

The first/hot manifold 706 may be connected to the inlet of the ORC heatengine and the outlet of the CSP via control valves 710 a and 710 b,respectively, to allow the heated working fluid to be transferredbetween the hot sides of pipes 702 and 704 and either the CSP or ORCheat engine. The second/cold manifold 708 may be connected to thereturn/outlet of the ORC heat engine and the inlet of the CSP viacontrol valves 712 a and 712 b, respectively, to allow the cold workingfluid to be transferred between the cold sides of pipes 702 and 704 andeither the CSP or ORC heat engine.

Valve 714 a on the hot side of pipe 702 and valve 714 b on the cold sideof pipe 702 may be opened to take pipe 702 out of the fluid circuit whenservice is needed and it may be closed to place pipe 702 in service byconnecting it to first/hot manifold 706 and second/cold manifold 708,respectively. Similarly, valve 716 a on the hot side of pipe 704 andvalve 716 b on the cold side of pipe 704 may be opened to take pipe 704out of the fluid circuit when service is needed and it may be closed toplace pipe 704 in service by connecting it to first/hot manifold 706 andsecond/cold manifold 708, respectively.

To monitor and control the temperature and pressure of the working fluidat the first/hot manifold 706 there are included temperature sensor 720and pressure sensor 722. To monitor and control temperature at thesecond/cold manifold there are included temperature sensor 724 andpressure sensor 726. As described above, the system maintains theworking fluid in the liquid state, so as the temperature of the workingfluid rises, the pressure in the system must be increased. Pressurecontrol is accomplished by increasing/decreasing the flow of the pumpsin the CSP working fluid loop and/or the ORC heat engine working fluidloop. The pressure may also be controlled by the control valves 710a/710 b and 712 a/712 b. Maintaining the working fluid in a liquid statecan also be accomplished by controlling the expansion system 750,described below.

Temperature sensors 730 and 732 are included with storage pipes 702 and704 respectively to measure temperatures at a plurality of locationsalong the lengths of the pipes. These temperature measurements may beused to control the movement of the separator pistons (not shown) asthey translate along the pipe as the storage pipes are charged anddischarged with the working fluid. The control scheme using thetemperature sensors will be described below.

There may also be included a purge and relief system 740 which includesa connection from the hot sides of pipes 702 and 704, via purge reliefvalve 742 a and check valve 744 a, to header 746. Purge and reliefsystem 740 also includes a connection from the cold sides of pipes 702and 704, via purge relief valve 742 b and check valve 744 b, to header746. The purge and relief system may be operated by a system controllerfor pressure relief to maintain system integrity in case of highpressure fault conditions

In addition, thermal storage system 700 may include an expansion system750 to accommodate the change in fluid volume between the fully chargedand fully discharged states. Expansion system 750 may operate in themanner that expansion system 614, FIG. 6 , operates as described above.The system includes control valve 752, which can connect the second/coldmanifold 708 to expansion reservoir. There is a pump 756 and check valve758 which may return the working fluid from the reservoir tank 754, asneeded. There is also a regulator which may provide a flow of nitrogenfrom tank 762 to expansion reservoir 754 via check valve to maintain thecorrect pressure in the expansion tank.

There is also shown a controller 770 which may control the operation ofthe various valves and pumps in order to charge and discharge thestorage system 740 as is described in more detail under the “storagesystem control” section below. The controller 770 may comprise anindustrial PLC, remote I/O, and supporting components to provide safetyand control functionality. The controller 770 may interact with aseparate or integrated higher level controller which may provide overallcontrol of the heat source, storage system and heat load. The operationof the higher level controller is described below in more detail withregard to FIGS. 16-18 .

FIG. 8 shows an example of a pipeline service or access port 800, whichmay be utilized in any of the pipe configurations described herein. Eachparallel storage pipe 802 may include an access port 800 including astandard weld neck flange 804 and a blind flange 806 at one end for thefloating separator piston installation and service access, and eitherthe same configuration or a closed cap at the opposite end. Both ends ofeach parallel pipe are equipped with a main port 808 for transferringpressurized water during charge and discharge cycles, and additionalports (not shown) for instrumentation such as fluid temperature,pressure, level, and potentially water chemistry.

The pipes disclosed herein are supported on industry standard pipelinesupports, with provisions to allow for axial movement as the pipe seesfull shutdown, charging, and discharging operating conditions. Theoverall pipe storage assembly may be insulated with rock or mineral woolor calcium silicate insulation, with a rainproof cover over theassembly. The insulation may be required to meet the systemscharge/discharge cycle efficiency specifications, and also limit thesurface temperature of the system as required for safety codecompliance.

All storage pipe components described in herein are standard partscommon in the pipeline industry, which are currently produced in volume.This is a significant advantage of the described thermal storage system.The storage pipes are largely fabricated from existing O&G andindustrial pipeline and pressure vessel technology and materials.Individual pipe sections may be welded together at the field site, orpartially assembled into the maximum transportable length in a factorysetting and then moved to the field site for final assembly. The lengthof each pipe assembly will be dictated by required thermal storagecapacity, number of individual pipes connected in parallel, and the sitespace limitations. A typical system might use as an example three 1 m(outer diameter) pipelines in parallel, each 250 m in length.

In addition to the storage pipes described herein, the manifolds,piping, and valves to connect multiple storage pipes in parallel, and totransfer the HTF between the hot and cold sides of the system may bestandard components. The pumps described herein may be designed foroperation with pressurized fluid on both the intake and outlet sideswhile operating at a low relative pump head.

Storage System Control

The storage control system may be implemented with software or firmwarerunning on controller 650, FIG. 6 , or controller 770, FIG. 7 , tomaintain the liquid state of the working fluid for pressurized waterstorage in a thermal storage system. It may also be used to coordinatecontrol of the floating separator pistons in the systems of FIGS. 6 and7 , as described in the remainder of this section. It should be notedthat these controllers may interact with or be implemented as part of asystem level controller as described below with regard to FIGS. 16-18 .

Closed liquid systems typically utilize an expansion system to protectagainst excessive system pressure changes due to fluid thermal expansionor other system variations during operation. Most expansion systems arepassive in nature, with an expansion tank charged to a minimum systempressure and with sufficient volume to help maintain a consistent systempressure during operation as system conditions change and the workingfluid expands and contracts. The described thermal storage systemsimilarly uses a closed pressurized working fluid, which increases anddecreases in density and volume as system conditions change and thethermal storage system is charged and discharged. The fluid volumeincreases as the thermal energy storage system charges, and converselydecreases as the storage is discharged. To accommodate the substantialchanges in fluid volume in the described storage system, which canexceed 40% in some cases when using water as the working fluid, anactively controlled expansion system is described.

With the storage systems described in this disclosure, e.g. systems 600and 700, active control of the expansion systems may be used. A valve ofthe expansion system may be used to bleed working fluid into anexpansion tank when pressure increases above a setpoint. Conversely, aninjection pump of the expansion system may be used to drive fluid backinto the system to increase pressure when below the setpoint. Thepressure setpoint is chosen based on the maximum fluid temperature so asto keep the fluid pressure sufficiently above its saturation pressure,and thus maintain the working fluid completely in the liquid state.Allowing the working fluid to flash into the vapor or gaseous state willdisrupt the storage system operation and introduce safety hazards forboth personnel and equipment.

The expansion tank is sized based on the total fluid volume in thesystem, and the maximum change in fluid density and volume across alloperation states including fully charged to fully discharged conditions.

-   -   Control: System pressure is measured at the outlet of the CSP or        other thermal energy supply, and used as a control input signal        to the pressure control system. A sufficient setpoint dead band        is maintained so as to avoid short cycling of the liquid state        control system.    -   Pressure>Setpoint: Bleed off HTF from the storage system through        an actuator-equipped control valve into the expansion tank. The        pressure in the expansion tank is maintained at a level above        the saturation pressure at the low side system temperature with        a typical nitrogen charged accumulator.    -   Pressure<Setpoint: When system pressure drops due to discharge        of the thermal storage system or other system conditions, the        injector pump is operated to return fluid from the expansion        tank into the storage system to increase the system pressure to        the desired setpoint.    -   An additional refinement of this control system can be the        addition of active monitoring of the fluid temperature exiting        the CSP or other thermal energy supply, and adjusting the        pressure setpoint to reflect the changing vapor pressure and        therefore minimum required system pressure to maintain a fully        liquid state for the working fluid. This addition may offer some        operating advantages in terms of system flexibility and safety.

It should be noted that the floating separator pistons described hereinmay be configured to move passively and independently of each other inthe various parallel vessel arrangements. Alternatively, controllers650/770 may control and coordinate movement of the floating separatorpistons in multiple storage pipes. The pistons may be controlled to movein synchronization, for example, within a parallel vessel arrangement.To synchronize the pistons, the controller of the thermal storage systemconfigurations according to this disclosure may determine each pistonposition within its respective vessel and then adjust the flow ofworking fluid to each vessel to keep the piston movements synchronized.This adjustment may, for example, be performed through control valves orpump speed adjustments. This action may be carried out in a number ofways. One method is to equip each parallel pipe with a flowmeter, anduse either a single system pump with flow control valves for eachparallel pipe or a separate variable speed pump for each parallel pipeto maintain equal flow to each pipe. This method is relatively easy toimplement, but it would need to account for potentially unequal pistonbypass leakage performance among the parallel pipes.

An alternate method of controlling movement of the floating separatorpiston in multiple storage pipes in parallel is to measure pipelinetemperature at multiple regular locations along the length of each pipe(as do temperature sensor 730/732 along pipes 702/704 of FIG. 7 ) and todetect the high differential temperature that occurs on each side of thefloating separator piston. This piston location input signal can then beused to control either flow control valves or variable speed pumps oneach pipe circuit to keep the parallel pistons in synchronization.

While in some cases it may be advantageous to synchronize the positionsof the pistons as they travel back and forth along their pipelines, insome instances it may be more of an advantage to charge and dischargethe multiple pipelines in a sequential fashion. Conversely, if thethermal energy losses from the storage pipes are significant there couldbe an operating advantage to maintaining a level of synchronizationamong the pistons in the multiple pipelines. The storage temperaturecould then remain more consistent among the pipelines because they fillat a similar rate and have a similar time in storage before discharge.

In another case, the inlet temperature to the heat load (e.g. an ORC)may be controlled by selectively adjusting flow rates from multiplestorage pipelines housing fluid at slightly varying temperature levels.Basically, output from cooler pipelines could be used to temper outputfrom hotter ones to manage the fluid temperature delivered to the heatload.

Floating Separator Piston Designs

A key aspect of the thermal storage system described herein is thedesign of a floating separator piston that can travel axially along thepipes while separating the changing hot and cool sections within thepipes as the storage system is charged or discharged using a commonworking fluid. The piston must move freely along the full length of theinterior region of the pipes in a response to small differences inpressure, typically below 5-10 psi, while providing sufficient sealingto separate the hot and cold sides of the working fluid stored withinthe storage vessel, even with storage vessels having interior surfaceswith variable roughness (i.e. non-honed surface). In other words, thefloating piston must engage with the inner surface of the pipe with asufficiently small amount of friction that it allows movement of thepiston in the pipe with low pressure differential, but also create asufficient thermal/fluid seal. These requirements are more challengingto realize with the floating large separator piston diameters (typically0.75 m-1 m) needed for this application. The piston must also withstandthe full temperature and pressure conditions of the storage system, andexhibit a long service life with minimal O&M support.

The various embodiments of the floating separator piston disclosedherein are designed to meet the following design requirements:

(1) withstand wide temperature variations, and be capable of operationat temperature in excess of 300° C.

(2) withstand high working fluid working pressures in excess of 100 bar.

(3) seal sufficiently to the pipeline inner surface to limit fluidbypass across the piston between the hot and cool regions of thepipeline storage volume.

(4) move back and forth along the pipeline under forces from pumping theworking fluid from one side to the other of the storage system.

(5) provide a sufficient thermal break or insulating properties to limitheat flow across the piston from the hot to the cold side.

(6) have a long working life, and be accessible for service if required.

There is shown in a perspective view in FIG. 9A and in a cross-sectionin FIG. 11 , a floating separator piston 900 in accordance with anaspect of the present disclosure. The piston 900 is cylindrically shapedand has at each end one or more sealing rings 904, and one or moreoutboard rider rings 906. In a central region of the piston, between thesealing rings 904, is a cylindrically shaped, low porosity, lowpermeability and compressible seal 902 (shown in phantom) seated in arecessed area 903 of about the circumference of the piston. This pistondesign must serve multiple functions, including to provide a sealagainst fluid bypassing between the piston ends, to provide thermalisolation, and to limit friction in order to allow the piston to move ortranslate along the pipe. The piston must realize these features whileproviding an acceptable service life.

The described compressible center seal is designed using a highlycompressible, limited porosity material with a substantial axialdimension instead of attempting to create a point seal using anon-porous material as in a typical piston ring constrained in a groove.One preferable version uses a multi-layer Kevlar felt material to createthe overall seal assembly. Another version may be made of a glass fibermaterial. The highly compressible nature of this seal design allows thepiston to operate inside a storage vessel fabricated from commerciallyavailable steel pipe with a variable surface roughness, as opposed to atypical cylinder bore used for most piston applications, which aretypically machined and honed to a close tolerance.

The seal performance can be tailored to the application by specificationof the uncompressed porosity of the seal material, the degree ofcompression of the seal assembly once the piston is installed in thepipeline, and the overall axial length of the seal. The resulting sealporosity is designed to limit the bypass leakage across the separatorpiston to no more than 5-10% of the storage system's overall thermallosses over a complete daily charge/discharge cycle. This level ofperformance can be achieved with various combinations of seal porosityproperties, degree of seal compression, and seal length.

This seal is applied around the center of the piston's body in a recessdesigned to both retain the seal 902 and to allow room to accommodateinitial seal compression. The seal 902 should extend over a significantportion of the overall axial length of the piston 900, increasing thepathway length for fluid to bypass the piston. The seal 902 typicallyextends along 50-90% of the length of the piston for best performance.

The compressible material of the seal 902 is compressed when the piston900 is installed in the pipe to create a prevailing pressure against thepipe's inner surface and to create an effective seal, but also allow formovement within the pipe, i.e. move freely in response to smalldifferences in pressure on either end of the piston, typically below5-10 psi. For example, the seal 902, when inserted into the pipe may becompressed to a thickness of 25-75% of its natural state. Therefore, inits uncompressed state the diameter of the seal would extend beyond theouter surface of the piston, such that when the piston is installed thein the pipe, the seal in the compressed state will have substantiallythe same outer diameter as the outer diameter of the piston.

The floating separator piston herein will typically have a symmetricaldesign; however, this is not a requirement of the piston according tothis disclosure.

Multiple material candidates could be considered for the compressibleseal 902. Kevlar felt is advantageous because it can withstand thetemperature and pressure conditions found in this thermal energy storageapplication. The Kevlar felt may be arranged in one layer or in aplurality of layers. A plurality of layers of Kevlar felt may bestitched together with Kevlar stitching and retained in a recessfabricated into the piston center section by banding straps. Alternateseal materials, such as felted glass may be selected depending on thedesign pressure and temperature requirements.

The compressible seal may incorporate a frame or armature to providestructure, aid in mounting the seal to the piston, and help maintain theseal shape during piston operation. As shown in FIGS. 9B and 9C, piston900 a includes seal 902 a which incorporates a support frame or armature920 that lends a degree of structure to the compressible seal assembly,aiding in the stable attachment of the seal 902 a to the piston 900 a.This armature may consist of a metal or composite material formed into arolled cylinder with regular openings (e.g. a mesh stainless steelsheet) or other features to allow the compliant seal material to beattached to both the inner and outer surfaces of the armature. The feltmaterial, e.g. Kevlar felt material, may be placed on both the interior,felt material 922, and exterior sides, felt material 924, of thearmature, and attached together to make the overall compressible seal902 a. Felt material 922 and 924 may be manually stitched onto armature920 using Kevlar stitching, rivets or other suitable methods.Alternatively the frame may be formed of a metal sheet with punchedholes, or a carbon mesh could be used.

As an example, when using a vessel with an ID of about 34″; the seal 902a may have a thickness of roughly 1.5″-2″ when uncompressed; and itmight be compressed to 25-75% of its original thickness depending onpipe tolerances and variations, and operating conditions. Theexpanded/mesh metal sheet of armature 920 may be in the range of 0.05″to 0.125″ thick.

The floating separator piston travels back and forth along the storagepipeline as the thermal storage vessel is charged or discharged, with asystem of piston seals that limits fluid blow past the piston-pipelinewall gap caused by pressure and thermal gradients experienced by thesystem. With the piston and pipeline oriented in a substantiallyhorizontal orientation, the effects of gravity on the piston and itspiston seal system are asymmetric and can result in uneven forces andwear over time, as well as increased seal friction at the lower sealarea, and increased leakage at the upper seal area. To mitigate thiseffect, the piston can preferably be designed to achieve a substantiallyneutral buoyancy when fully submersed in the liquid working fluid. Whilethe buoyancy would vary somewhat as fluid and piston temperaturesfluctuate during operation, a substantially neutral buoyancy state forthe piston offers a definite operating advantage.

This neutral buoyancy state for the floating separator piston 900 can beachieved in a number of ways. A void or internal chamber encompassingeither the entire or a portion of the piston interior region 908 can beintroduced into the piston structure that is isolated from the systemworking fluid, variously evacuated to create a vacuum; filled with airor a relatively nonreactive gas such as nitrogen; filled with a lightmaterial such as foamed glass or metal; or one or more buoyant objectssuch as a gas filled enclosure(s) introduced to the interior of thepiston The level of buoyancy to be introduced into the piston would bedesigned to roughly offset the gravitation forces on the piston, with agoal of reducing the gravitation loads on the piston and its seals toless than 20% of loads on a piston with no buoyancy. The piston 900 mayfurther include insulating materials, thermal breaks, or sealed chamberswithin interior region 908 to decrease axial thermal conduction throughthe piston.

Pressure relief valves can be incorporated into the piston 900 to limitthe maximum pressure differential across the piston. This providesadditional system safety by limiting the net moving force on the pistonduring system fault conditions such as control anomalies, and alsoprevent pressure buildup and rapid piston movement if the piston getsstuck in the pipeline. Features can be built into the piston to alloweasier extraction from the pipeline if the piston becomes lodged duringoperations, or to retrieve the piston during routine service intervals.Passive or active instrumentation can be incorporated into the piston tomonitor leakage bypass to determine seal performance, and to monitor thelocation of each piston in the overall system.

One or more sealing rings, 904 may be included near each end of thepiston and may be made of any suitable metal, polymer, or compositematerial formed into a ring and mounted in a retaining slot or featurein the piston body. The piston rings 904 provide a seal against axialfluid bypass through the piston-pipeline interface. FIG. 10 shows twoexemplary segmented piston ring designs 1002 and 1004, which may be usedin accordance with embodiments of the present disclosure. The pistonrings may be retained in circumferential slots in the piston body. Therings 1002 and 1004, are designed to expand against the inner surface ofthe pipe and provide a sealing function to limit fluid bypass around thepiston between the hot and cold pipe regions. It is to be understoodthat either one or both ring designs 1002 and 1004 could be used assealing rings 904. In addition, any other sealing ring known to a personhaving skill in the art may be used.

The sealing rings may be formed of a plurality of arc segments, such assegments 1002 a-1002 c of sealing ring 1002 and segments 1004 a and 1004b of sealing ring 1004. The segments are connected end to end using, forexample, an articulating joint, such as joints 1006 a and 1006 b forsealing ring 1002 and joint 1008 a for sealing ring 1004. Together thesegments form a complete ring that is mounted in a corresponding pistonring groove feature on the piston. This segmented ring design carries anumber of advantages:

(1) The piston and corresponding pipeline inner diameter are quitelarge, typically on the order of 1 m in diameter or larger. Fabricationof a one-piece piston ring of this diameter is expensive and difficultbut may be a viable option in certain circumstances. The arc segments ofa multi-piece ring are smaller and easier to fabricate using a varietyof methods, including machining, roll forming, and particularly 3-Dprinting methods, and are therefore generally preferred to thesingle-piece ring.

(2) Clearance can be designed into each articulating joint between thering sections, providing a controlled amount of compliance in thecompleted ring to adapt to variations in the matching pipeline diameterand ovality.

An exemplary sealing ring may be divided into eight sections, but thering could alternately be configured with any practical number ofsections based on the capabilities and associated cost of the ringmaterial and the fabrication method used. Dividing the sealing ringsinto eight sections for a one (1) meter nominal ring creates sectionsthat can readily be fabricated using existing 3D printing methods.

The ring sections can be fabricated using a variety of materials andusing a variety of fabrication methods. 3D printing can be used tocreate ring sections out of a thermoplastic polymer, such as Polyetherether ketone or Polyether ketone. Alternatively, the ring sections couldbe formed from steel, stainless steel, Inconel, and a variety of bronzealloys depending on the pressure and temperature requirements of thespecific application.

The ring sections can be designed to incorporate spring elements, suchas spring element 1010 mounted in a recess of segment 1002 b of sealingring 1002, to provide a controlled prevailing pressure against thepipe's inner wall, allowing effective sealing while accommodatingpipeline ID variations and seal wear over time. In one embodiment, thespring element used to position the ring against the pipeline wall canbe a single piece linear wave spring, formed into a circle and retainedin a matching recesses on the inner circumference of each ring segment.In another embodiment, each ring segment has its own spring element,with features incorporated into each ring section to retain itsrespective spring element. The spring elements may be any suitable type,including wave, coil, leaf or zig-zag. While not shown, the varioustypes of spring elements may be incorporated into recesses formed in theinner diameter of sealing ring 1004.

While a sealing ring, including a plurality of segments is describedhere, it is expressly contemplated that the sealing ring 904 may beformed as a one-piece split ring that can be sprung out to mount in amatching slot in the piston. The one-piece ring may then contract tostay retained in the piston slot, similar to conventional internalcombustion engine piston rings. The one piece sealing ring formed ofsteel may be designed to have a slightly larger diameter than the innerdiameter of the recess in the piston surface so that it may be retainedin the recess, and it provides a prevailing spring force against theinner surface of the pipe once the piston is installed in the interiorof the pipe.

The piston 900 may also include one or more rider rings 906. The riderrings 906 may be located at each end of the piston 900, outside of thesealing rings. The primary function of the rider rings 906 is to centerand locate the piston within the pipe bore and to take the primarygravity and other asymmetric loads on the piston to reduce wear of thesealing rings. The rider rings 906, therefore, protect the sealing ringsfrom excessive forces and wear. Exemplarily, the rider rings 906 may befabricated from a Ni—Al-Bronze alloy.

Because the rider rings are not designed for a sealing function, but asa locating and wear pad element, alternatively multiple rider pads maybe installed on each end of the piston to locate it within the pipe andprovide the same functions as the rider rings. The rider pads canreadily be fabricated from stock shapes with minimal material waste andlow cost. FIG. 12 shows another piston design 1200 in accordance with anembodiment of the present disclosure which utilizes rider pads. Thepiston 1200 includes segmented sealing rings 1204 and a compressible lowporosity seal 1202 in the center region of the piston. The seal 1202 maybe constructed similar to the seal 902 of piston 900 described above.Further, the segmented rings 1204 may be constructed similar to thesealing rings 904 of piston 900. Instead of using rider rings 906 ofFIG. 9A, rider pads 1206 may be included around the circumference ofeach end of piston 1200 to perform the same function as the rider rings.

FIG. 13 shows another piston design 1300 in accordance with an aspect ofthe present disclosure. Piston 1300 includes a compressible, lowporosity member 1302 made of glass felt, which acts as the seal betweenthe hot and cold parts of the working fluid similar to the comparablecompressible members of FIGS. 9 and 12 . However, piston 1300 includesspring-loaded, wedge-shaped members 1304 and 1305, each mounted by meansof a hinge or pivot distally to the piston, and located about thecircumference of each piston end that expand to fill in the gap betweenthe piston 1300 and the interior wall 1306 in which the piston isdisposed. The wedge shaped members 1304 and 1305 pivot individually toconform to the gap between the piston 1300 and the pipe 1306. The piston1300 also has spring-loaded rollers 1308 and 1310 which connect with thewall of the pipe 1306 and perform the same function as do the riderrings and rider pads of the above-described piston embodiments.

FIG. 14 shows a piston 1400 in accordance with another embodiment of thepresent invention. Piston 1400 also has spring-loaded rollers 1404 and1405, which perform the same function as do the rider rings and riderpads of the above-described piston embodiments. Piston 1400 alsoincludes a mesh screen 1402, which acts as the compressible, lowporosity member, and low permeability material. The rollers 1404 and1405 are spring-loaded and connect with the wall of the pipe 1406. Themetallic mesh 1402 acts as a separator between the hot and cold sides ofthe working fluid.

FIG. 15 shows a piston 1500 in accordance with yet another embodiment ofthe present invention. The piston 1500 has a magnetic core (not shown)surrounded by compressible ferrofluid material 1502. The ferrofluid actsas a separator between the hot and cold sides of the working fluid.Although not shown, spring-loaded rollers, rider rings or rider padscould be utilized with this embodiment.

In any of the piston designs described above, spring loader rollers,rider pads and/or rider rings may be utilized. In addition, any of thefloating piston designs may be configured to move within the storage“pipe” passively in response to small pressure differential between theends of the piston (i.e. 5-10 psi) and fluid flow caused by pumping thefluid between the opposing hot and cold sections. In addition, in otherembodiments, the piston itself may be equipped with a drive mechanism totravel under its own power in an axial direction along the interior ofthe pipe, effectively acting as a positive displacement pump to drivethe fluid between the opposing hot and cold sections.

The piston designs may incorporate insulation properties to minimizeheat transfer through the piston between the hot and cold sides of thethermal storage system. They may include a system of seals to minimizeleakage of the working fluid around the piston from between the hot andcold sides of the thermal storage system. In addition, the piston may bedesigned with features to keep the surfaces of the pipeline operatingregion clear of deposits and accumulated buildup of the system'soperating life. These features could replicate functionality found inO&G pipeline “pigs” used to periodically clean and inspect pipelinesystems.

Overall Control System

The control system for a renewable energy storage and dispatch systemdescribed herein may consist of, for example, an industrial PLC, remoteI/O, and supporting components to provide safety and controlfunctionality for the thermal storage system, and to interface with thecontrols of the heat source (e.g. a CSP solar system) and of the thermalload (e.g. ORC heat engine system) to create an overall control systemfor generation, storage, and use of thermal energy.

The control system controls overall operation under all conditions,managing transitions between the defined operating states for thesystem. A simplified state diagram 1600 for the control system is shownin FIG. 16 . The control system may, for example, be used to control therenewable energy storage and dispatch system 600 shown and describedabove with reference to FIG. 6 . Of course, this control system couldreadily be adapted to operate any of the thermal storage systemembodiments described herein.

Within each operating state of state diagram 1600, a defined sequence ofconditional operations is carried out by the system controller which arespecific to that operating state. Data from temperature, pressure, flow,level, and potentially other sensors are read and recorded, and thesystem pumps, valves and potentially other output devices are commandedto respond to start, stop, and manage the charge and discharge of thestorage system. In addition, the storage control system communicateswith both the thermal energy sources (solar, geothermal, biowaste etc.)and the thermal loads (ORC heat engine, industrial heat, etc.)subsystems to maintain a complete dispatchable and controllable energysystem.

In state 1 (off state), shown in block 1602 of state diagram 1600, theCSP solar field is in an inactive state and the ORC heat engine isturned off. Additionally, in this state a no system fault detectedsignal is received. In state 2 (Warm up), block 1604, the CSP solarfield is activated, the ORC heat engine is maintained in the off state,and the CSP solar field brings the working fluid of the storage systemup to its normal operating temperature. In state 3 (Normal daytime)block 1606, the CSP solar field is active, the ORC heat engine is onlineand the storage system is charged by the CSP solar field, and the systemwaits for a discharge demand from the ORC heat engine. The normaldaytime mode continues with charging and discharging cycles as required.In state 4 (Normal night) block 1608, the solar field is inactive andthe ORC heat engine is active. The transition to Normal night wouldoccur after the thermal storage system is charged by the CSP solar fieldone last time Normal daytime state.

When operating either in the Normal daytime state or the Normal nightstate, if a service shutdown is activated, the system transitions tostate 5 (Service shutdown) block 1610, where the CSP solar system andthe ORC heat engine are inactivated, but the system controls remainactive. When operating either in Normal daytime state or Normal nightstate, if a fault shutdown is activated, the system transitions to state6 (Fault shutdown) block 1612, where the CSP solar system and the ORCheat engine are inactivated, and the system controls are inactivateduntil a fault clearance signal is received.

A simplified example of pump and control valve actions matrix 1700, FIG.17 , is shown for several system states of state diagram 1600 of FIG. 16. Matrix 1700 illustrates example controller output commands forcontrolling various pumps P1-P3 and control valves CV1-CV7 for therenewable energy storage and dispatch system 600 of FIG. 6 during theStart-up, Day, and Night states.

FIG. 18 shows an example of control system flow diagram 1800 for thedefined Normal daytime operational state (state 3) block 1606 of thestate diagram of FIG. 16 . In step 1802, it is determined if the ORCheat engine has been dispatched to supply electricity to its loads. Ifthe heat engine has been dispatched, the system proceeds to step 1804 todetermine if the current output from the CSP solar system exceeds theORC heat engine demand. If the heat engine has not been dispatched atstep 1802, the current output from the CSP solar system is directed tothe thermal storage system to charge the system at step 1806. Referringto line 1710 of matrix 1700, FIG. 17 , and system 600, FIG. 6 , in thisstep the CSP solar system 620 is on, pump 606 (P1) is activated, controlvalves 619 b (CV2) and 619 d (CV3) are open, and control valves 619 c(CV4) and 619 e (CV5) are closed.

Referring again to step 1804, if the current output from the CSP solarsystem is determined to exceed the ORC heat engine demand, at step 1808the demanded amount of thermal output from the CSP solar system isprovided to the ORC heat engine and the remaining/excess thermal outputis directed to the thermal storage system to charge the system.Referring to line 1720 of matrix 1700, in this step, the CSP solarsystem 620 and the ORC heat engine 640 are on, pumps 606 (P1) and 618(P2) are activated, and control valves 619 b (CV2), 619 d (CV3), 619 c(CV4) and 619 e (CV5) are open.

If at step 1804, the current output from the CSP solar system isdetermined to not meet the ORC heat engine demand, the system proceedsto step 1810 where it is determined if the thermal storage system hasenough thermal energy stored to meet the CSP solar output shortfall. Ifat step 1810 it is determined that the storage system has enough storedthermal energy, in step 1812, the thermal storage system supplies enoughthermal energy in combination with the CSP solar system to support thedemand of the ORC heat engine. Referring to line 1730 of matrix 1700, inthis step, the CSP solar system 620 and the ORC heat engine 640 are on,pumps 606 (P1) and 618 (P2) are activated, and control valves 619 b(CV2), 619 d (CV3), 619 c (CV4) and 619 e (CV5) are open.

If at step 1810 it is instead determined that the storage system doesnot have enough stored thermal energy, in step 1814, the thermal storagesystem is charged by the CSP solar system and does not supply to the ORCheat engine. Referring to line 1740 of matrix 1700, in this step, theCSP solar system 620 is on and the ORC heat engine 640 is off, pumps 606(P1) and 618 (P2) are activated, and control valves 619 b (CV2) and 619d (CV3) are open and control valves 619 c (CV4) and 619 e (CV5) areclosed.

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inany appended claims.

What is claimed is:
 1. A thermal energy storage system for storing thermal energy produced by a heat source and for supplying the thermal energy to a thermal load, the thermal energy storage system comprising: a working fluid configured to store the thermal energy and transfer the thermal energy between the heat source and the thermal load; a vessel configured to store the working fluid; the vessel having a first end, a second end, an interior region, and a floating piston located in the interior region to separate a hot portion of the working fluid towards the first end from a cold portion of the working fluid towards the second end; wherein the floating piston comprises a piston body having a first end, a second end, and a central region, and a compressible member which is disposed in the central region of the floating piston and which is configured to engage with an inner surface of the vessel; and wherein the compressible member has a porosity level that results in a thermal loss due to leakage of the working fluid from the first end of the piston to the second end of the piston of no more than 5% of an overall thermal loss in the thermal energy storage system; a first manifold configured to be thermally coupled to an output of the heat source and to an input of the thermal load and fluidly coupled to the interior region proximate the first end of the vessel; a second manifold configured to be thermally coupled to an input of the heat source and an output of the thermal load and fluidly coupled to the interior region proximate the second end of the vessel; and a controller configured to maintain the working fluid in a liquid state.
 2. The thermal energy storage system of claim 1, wherein the first manifold is thermally coupled to the output of the heat source by way of one of a first heat exchanger or a direct fluid coupling; and wherein the first manifold is thermally coupled to the input of the thermal load by way of one of a second heat exchanger or a direct fluid coupling.
 3. The thermal energy storage system of claim 2, wherein the second manifold is thermally coupled to the input of the heat source by way of one of the first heat exchanger or a direct fluid coupling; and wherein the second manifold is thermally coupled to the output of the thermal load by way of one of the second heat exchanger or a direct fluid coupling.
 4. The thermal energy storage system of claim 1, wherein the thermal heat source is one or more of a concentrating solar power system, a geothermal system, a biomass system, a waste-to-energy system, and an industrial heat recovery system and wherein the thermal load is one or more of a heat engine and/or an industrial process heat load.
 5. The thermal energy storage system of claim 1, wherein the working fluid comprises one or more of water, water mixed with one or more additives, oil, refrigerants, and molten salts.
 6. The thermal energy storage system of claim 1, wherein the working fluid is water and the controller is configured to maintain the hot portion of the working fluid at a temperature from about 200 to 360 degrees C. and to maintain the cold portion of the working fluid at a temperature from about 80-170 degrees C.
 7. The thermal energy storage system of claim 6, wherein the controller is configured to maintain a pressure of the working fluid between 225 psi (15 bar) and 2700 psi (190 bar) to maintain a liquid state.
 8. The thermal energy storage system of claim 1, wherein the vessel is disposed in a substantially horizontal direction relative to a surface on which the thermal energy storage system is disposed; wherein the first end of the vessel is positioned at a first height above the surface on which the thermal energy storage system is disposed and the second end of the vessel is positioned at a second height above the surface; and wherein the first height is greater than the second height.
 9. The thermal energy system of claim 8, wherein a difference between the first height and the second height results in the at least one vessel is oriented at an angle of between 0.25 and 2 degrees relative to the surface.
 10. The thermal energy storage system of claim 1, wherein the vessel comprises steel and is insulated.
 11. The thermal energy storage system of claim 1, wherein the vessel comprises a plurality of vessel sections joined together via welding; wherein each of the vessel sections is 40 to 80 feet in length and 24 to 48 inches in diameter.
 12. The thermal energy storage system of claim 1, further including a first pump connected between the heat source and the vessel to circulate the working fluid between the heat source and the vessel.
 13. The thermal energy storage system of claim 12, further including a second pump connected between the vessel and the thermal load to circulate the working fluid between the vessel and the thermal load.
 14. The thermal energy storage system of claim 7, further including a thermal expansion system fluidly coupled to one of the first or second manifolds to accommodate a change in working fluid volume.
 15. The thermal energy storage system of claim 14, wherein the thermal expansion system includes an expansion tank and an injection pump; and wherein the controller directs the working fluid from one of the first or second manifolds into the expansion tank when the pressure of the working fluid exceeds a setpoint pressure and the controller causes the injection pump to drive the working fluid from the expansion tank to one of the first or second manifolds when the pressure of the working fluid falls below the setpoint pressure to maintain the working fluid in the liquid state.
 16. The thermal energy storage system of claim 1, wherein the vessel comprises one or more of pipes, tubes, or conduits.
 17. The thermal energy storage system of claim 1, wherein the controller is configured to control movement of the floating piston.
 18. The thermal energy storage system of claim 1, wherein the compressible member is compressible to a thickness of between 25-75% of its original thickness.
 19. The thermal energy storage system of claim 1, wherein the compressible member has a length of at least 50-90% of a length of the piston body.
 20. The thermal energy storage system of claim 1, wherein the compressible member includes one or more of synthetic fiber, glass, a ferrofluid, or a metallic material.
 21. The thermal energy storage system of claim 1 wherein the compressible member engages with an inner surface the vessel with an amount of friction that allows movement of the floating piston in the vessel with a pressure difference of not more than 10 psi between the first end of the piston to the second end of the piston.
 22. The thermal energy storage system of claim 21, wherein the inner surface of the vessel has a variable roughness.
 23. The thermal energy storage system of claim 1, wherein the floating piston has a neutral buoyancy state in the working fluid.
 24. The thermal energy storage system of claim 23, wherein the piston body includes an internal chamber and wherein the internal chamber is evacuated to create a vacuum or includes one or more of air, a nonreactive gas, a foamed glass or a metal.
 25. The thermal energy storage system of claim 1, wherein the vessel is disposed in a substantially horizontal direction relative to a surface on which the thermal energy storage system is disposed. 